This invention relates to an all natural, starch-based corrugating adhesive providing water resistance without containing water resistance additives or chemical crosslinkers. More particularly, this invention is directed to a corrugating adhesive composition comprising an aqueous dispersion of a selected gelatinized carrier starch and a raw ungelatinized starch wherein the carrier starch has an amylose content of greater than 60% by weight and the cooked or gelatinized amylose content is at least about 15% by weight of the total starch content in the adhesive.
The procedure employed in the production of corrugated paperboard usually involves a continuous process whereby a strip of paperboard is first corrugated by means of heated, fluted rolls. The protruding tips on one side of this fluted paperboard strip are then coated with an adhesive, and a flat sheet of paperboard, commonly known in the trade as a facing, is thereafter applied to these tips. By applying heat and pressure to the two paperboard strips thus brought together, an adhesive bond is formed between the strips. The above-described procedure produces a single-faced board in that the facing is applied to only one surface thereof. If a double-faced paperboard in which an inner fluted layer sandwiched between two facings is desired, a second operation is performed wherein the adhesive is applied to the exposed tips of the single-faced board and the adhesive-coated tips are then pressed against a second facing in the combining section of the corrugator under the influence of pressure and heat. The typical corrugating process and the operation and use of corrugators in general are described in U.S. Pat. Nos. 2,051,025 and 2,102,937 issued on Aug. 18, 1936 and Dec. 31, 1937 respectively to Bauer.
A major concern in such corrugating processes is the selection of the appropriate adhesive, as its properties will affect not only the strength and stability of the final corrugated product, but also the parameters, such as corrugator speed, within which the process can be operated. Thus, such adhesives are chosen in light of the specific requirements of the process and the properties desired in the ultimate corrugated paperboard. The adhesives most commonly used in corrugating are starch-based adhesives which are popular because of their desirable adhesive properties, low cost and ease-of preparation.
The most fundamental of starch corrugating adhesives is an alkaline adhesive which is comprised of raw ungelatinized starch suspended in an aqueous dispersion of cooked starch (carrier). The adhesive is produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a primary mix of gelatinized or cooked carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch, borax and water to produce the fully formulated adhesive. In the corrugating process, the adhesive is applied (usually at between 25.degree. and 55.degree. C.) to the tips of the fluted paper medium or single-faced board, whereupon the application of heat causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and tack and formation of the adhesive bond.
It is often desired or necessary in the manufacture of corrugated paperboard that the adhesive yield water resistant bonds which can withstand extended exposure to high humidity, water, melting ice and the like. A number of approaches have been devised to produce water resistant corrugating adhesives. One method involves preparation of an acidic, starch-based adhesive wherein urea-formaldehyde together with an acidic catalyst is added to the composition to produce a crosslinked starch and provide water resistant bonds in corrugated board. Another more commonly followed method involves the use of alkaline curing starch-based adhesives wherein formaldehyde-based chemicals are added as crosslinking additives to produce water resistant bonds.
It has been known for many years that a corrugating adhesive whose carrier portion is prepared from a high amylose starch (i.e., at least 35 to 40% amylose) is superior to one prepared from pearl starch, which contains about 27% amylose, because a carrier can be produced having improved rheological and film-forming properties, and increased moisture resistance. However, despite this knowledge, the ability to provide useful water resistant starch-based corrugating adhesives has been primarily directed to compositions containing synthetic additives, formaldehyde-based crosslinkers or other starch crosslinking agents. This is true with alkaline corrugating adhesives containing high amylose carrier starches which provide little or no water resistance when used without crosslinking additives. In other words, to get significant or reasonable amounts of water resistance even when using high amylose starches, crosslinking additives have been required.
Starch-based corrugating adhesives containing high amylose carrier starch and providing improved and superior green bond strength and tack are disclosed in U.S. Pat. No. 4,787,937 issued Nov. 29, 1988 to C. Leake. High amylose starches are also disclosed in U.S. Pat. No. 4,912,209 issued Mar. 27, 1990 to C. Leake et al., as being useful carrier starches in corrugating adhesives providing faster bond development and higher corrugator running speeds.
The use of high amylose starch in water resistant corrugating adhesives has been disclosed in U.S. Pat. Nos. 3,284,381 issued to L. Hickey et al. on Nov. 3, 1966 and 3,532,648 issued to D. Wilhelm on Oct. 6, 1970. However both references require the use of chemical crosslinkers commonly based on formaldehyde to provide desired water resistance. Other patents which disclose the use of crosslinking agents or other water resistance additives include U.S. Pat. No. 3,728,141 issued Apr. 17, 1973 to D. Ray-Chaudhuri et al. involving the use of a diacetone acrylamide-formaldehyde condensate, U.S. Pat. No. 3,944,428 issued Mar. 16, 1976 to J. Schoenberg et al. which is directed to an acetoacetamide-formaldehyde condensate and U.S. Pat. No. 4,009,311 issued Feb. 22, 1977 to J. Schoenberg which shows the reaction product of a ketone, formaldehyde and a secondary amine as a crosslinking agent.
More recently, due to the uncertainty of the safety of formaldehyde, efforts have been made to reduce the levels of exposure to formaldehyde. U.S. Pat. No. 4,366,275 issued on Dec. 28, 1982 to M. Silano et al. employs an acetone-formaldehyde condensate crosslinking additive low in free formaldehyde in an alkaline corrugating adhesive. U.S. Pat. No. 5,079,067 issued on Jan. 7, 1992 to S. Willzing discloses that the levels of free formaldehyde in alkaline starch containing corrugating adhesives can be reduced by reacting the free formaldehyde with a nitrogen base and urea.
Other patents disclose water resistant corrugating adhesives which use additives that are formaldehyde free. These patents include U.S. Pat. No. 4,775,706 issued Oct. 4, 1988 to C. Iovine et al. which uses a latex polymer of a halohydrin quaternary ammonium monomer, U.S. Pat. No. 5,055,503 issued Oct. 8, 1991 to C. Leake et al. disclosing a polysaccharide graft copolymer containing a reactive aminoethyl halide group and U.S. Pat. No. 5,190,996 issued Mar. 2, 1993 to M. Foran et al. showing an epihalohydrin-amine condensate crosslinking agent.
Despite the advances made to reduce or eliminate the level of formaldehyde exposure, the corrugating industry is still searching for effective means of providing water resistance to corrugated paperboard products using formaldehyde-free alkaline curing starch-based adhesives.
Accordingly, it is an object of the present invention to provide a water resistant, all natural (i.e. without crosslinkers), formaldehyde free, alkaline starch-based adhesive composition which is useful in the manufacture of corrugated paperboard and in which the water resistance is developed solely through the starch and does not rely on the addition of synthetic crosslinkers.